In the blue corner, with experience in Black Mesa and in his most recent games, he fought against a totalitarian race of aliens named the Combine. He is what's known as a silent protagonist and he doesn't talk much. It's Gordon Freeman from the Half-Life series!
And in the red corner, a man who knew war from the day he was born, and fights against an evil cult of aliens. He talks more than Gordon, and his favorite color is green. It's Master Chief from the Halo series!

I look forward to explain why I think Master Chief "John" would most certainly beat Gordon Freeman in a hand-to-hand fight where neither has any ally and (if they use guns I'm almost 100% master chief would win) but let's assume it's just plain mixed martial arts MMA, ultimate fighting championship (UFC) style fighting whereby it is civilised (no biting, eye gouging nor groin touching in any way) and it is a fight to the death where they do not drink, do not eat from the moment the fight begins until the moment one, or both, falls dead. Both are gully ready with the ideal amount of nutrients for either of them to be at their peak and neither has an ailment of any kind.

I request that we send Master Chief back in time (since he isn't born yet in 2012) at the age of 41, however I will also accept 45 since both ages are covered in his life story. I would like to consider Gordon Freeman to be around 35 years, since he was born between 1982-1973 and actually the story of his life only goes up to 27.

I would request that during it, neither competitor wears a protective clothing of any kind since Master Chief's suit would cause extreme bias to my side of the debate, the con.

First let me introduce Master Chief "John"

"He is neither the smartest nor the fastest nor the strongest of the Spartans. But he is the bravest - and quite possibly the luckiest. And in my opinion, he is the best."
— Dr. Catherine Halsey to Cortana

Physical description
GenderMale
Height208 centimetres (6 ft 10 in) [1] (without armor)
Weight179 kilograms (390 lb)[1](without armor)
Age By technical human Earth years, John is approximately 41 years old in 2552, and 45 years old in 2556.

He is referred to universally by humans simply as the "Master Chief", "Chief" for short and "Sierra-117" but his actual name is John (later changed to John-117).

At six, he was identified as one of 150 preliminary candidates for the SPARTAN-II Program. John was an ideal physical and mental candidate, standing a head taller than the majority of his schoolmates, having greater physical proportions, possessing greater strength, exhibiting superior reflexes, and having an aggressive drive for success.

At the age of fourteen, John underwent the dangerous SPARTAN-II augmentation procedures. John was one of 33 Spartans who made it through the process unscathed. At only fourteen years old, it is said that John had a body of an eighteen-year old Olympic athlete; the augmentation process gave the Spartans faster reflexes, greater strength, enhanced eyesight, and made their bones nearly unbreakable.

As a child, John strove to win at any situation, at all costs. As his career progressed, he transformed his desire to win personal glory to that of victory for his squad. John is tenacious to complete any mission at hand, and often disregards his personal safety in order to win. Indeed, he has often emerged victorious from situations many would consider impossible. Still, even a Spartan is not devoid of fear, but John simply acknowledges it and puts it aside, while never openly showing it.

John is also known as a man of his word and will strive to keep any promise he makes, even at great personal risk. As he was forced to leave Cortana behind on High Charity, John promised to come back for her after dealing with the Prophet of Truth. Cortana chided him not to make a promise when he knew he couldn't keep it. However, despite incredible odds, John did keep his promise, as he fought his way through the Flood into High Charity to retrieve her.

Now I shall use the information I just supplied, regarding Master Chief in support of his winning of the battle.

Firstly, standing at 6ft 10, I extremely doubt Gordon Freeman is nearly as close to 7 foot as that (if he is even 6 foot). I couldn't find the height of Gordon Freeman anywhere so am not sure but am definitely 99% sure that height is definitely in Master Chief's Favour. At the age of 41 has has more experience in battlefield environments than the supposed 35 year old Gordon Freeman. Gordon Freeman was originally a nerd who got a Ph.D. in theoretical physics from MIT. He got this at the age of about 27 so up till then was probably that skinny guy at school who bullies would pick on as an easy target. From the age of six Master Chief had already been spotted as a ferociously talented spartan, standing a head taller than the majority of his schoolmates, having greater physical proportions, possessing greater strength, exhibiting superior reflexes, and having an aggressive drive for success. This probably meant that if any 'gordon freeman' type person came across the 'master chief' guy at school, the Master Chief would be the bully in any scenario. This shows the dominance in a very blatant light.

Additionally, Master Chief has undergone the augmentation process that gave the Spartans faster reflexes, greater strength, enhanced eyesight, and made their bones nearly unbreakable.

He is renowned for his strike of 'luck' in achieving almost impossible feats. Thus, even if Gordon was fast and beat Master Chief up, I think he's definitely find a gap in Freeman's defence and do the impossible and turn the fight around on the physics nerd.

He is also renowned for his must-win attitude that will cause extreme psychological benefits such as adrenalin and fear of losing meaning he gets the physical edge that a person without such an urge to win and fear of losing would get so early on in the fight since adrenalin release wouldn't occur for them.